Project Summary How are V(D)J recombination events involved in generating oncogenic chromosome translocations underlying some leukemias and lymphomas? Many proposals have been advanced. Some events appear to involve joining of RAG-generated DSBs to breaks made by other mechanisms at the partner loci. A few years ago, we proposed that errors the choice of repair pathway to heal the DSB could lead to aberrant joining events. Our recent data now reveal that specific mutations in either RAG1 or RAG2 abrogate pathway choice and also destabilize the post-cleavage complex in vitro. Our preliminary data implicate some of these mutations in RAG-induced oncogenic translocations in murine lymphomas. These same mutations are associated with increased alternative NHEJ in lymphocytes in vivo. Together, these data support the following hypotheses, which we will test in the proposed experiments. Hypothesis 1: Pathway choice control plays a critical role in maintaining genomic stability, and is maintained in V(D)J recombination by the RAG post-cleavage complex. Decreased stability of the RAG postcleavage complex (resulting from any of a number of causes) allows increased availability of the coding and/or signal ends to inappropriate joining pathways, facilitating formation of aberrant V(D)J recombination products, including oncogenic chromosome translocations. Hypothesis 2: Abrogating pathway choice control allows alternative NHEJ to emerge as a mutagenic repair pathway. Understanding control of pathway choice and the consequences of disabling this regulatory mechanism in the context of V(D)J recombination will illuminate a question that has remained unanswered for almost 30 years: how do the oncogenic translocations that occur in developing lymphocytes arise? The knowledge we gain from the proposed studies is also likely to help us approach this important question in other contexts. If, in the proposed studies, our RAG mutants which deregulate pathway choice allow us to observe frequent oncogenic rearrangements in the context of an intact classical pathway, we will be able to establish that alternative NHEJ can indeed compete with the classical joining mechanisms. This result would imply that control of pathway choice is imposed, by some mechanism, in other DSB repair situations that do not involve the RAG proteins.